1. Field of the Invention
The present invention relates to an improved steel used for producing gears, and more particularly to improvements of such a steel, in machinability as well as strength and toughness.
2. Discussion of the Prior Art
Many types of gears have been used for various machines, devices, units and other equipment for transmitting power or motions. Automotive vehicles as transportation vehicles, for example, use a number of gears in steering and transmission systems, differential units and other devices. Improvements in properties of such gears have been increasingly desired, keeping up with a recent growing need for enhancing the quality and capability of the various machines, devices and units. In the fields of manufacture of automotive vehicles, it is desired to further improve the mechanical strength of the gears and other power transmission components, in an attempt to satisfy increasing requirements for enhanced performance, increased horsepower and reduced weight. There is a similar tendency in connection with steels for many mechanical structures.
In view of the industrial tendency indicated above, one of the applicants of the present application and other co-inventors proposed improved steels which make it possible to produce gears having high degrees of strength, toughness and reliability, as disclosed in laid-open Publications 60-21359 and 60-243252 of unexamined Japanese Patent Applications 58-128787 and 59-96600. Each of the proposed gear steels includes suitable amounts of C, Mn, S, Cr, Al, N, Si and P, and further includes specified amounts of Ni and Mo. These gear steels were confirmed to have advantages over the known steel materials such as SCr 420, SCM 420 and SNCM 420 (according to the Japanese Industrial Standard). In the high-strength steels proposed in the above two Publications, up to 0.030% or 0.020% of sulfur (S) is included, as an element for effectively improving the machinability of the steels.
Gears are generally formed such that stresses at the root of the teeth are exerted in a direction perpendicular to the direction in which materials for the blanks or workpieces of the gears are rolled. If the steel for a gear contains a certain amount of sulfur (S) for improving the machinability, as indicated above, sulfides or other substances exist extending in the rolling direction of the material, causing the material to be comparatively easily subjected to fatigue and impact destruction.